Am using one GLCD and U8glib lib.
So far everything is working OK but there is one problem. If arduino is powered over USB then brightness of LCD is normal ( blue background, slightly whiter foreground and you can easy see text on LCD.
But when i switch to 12V arduino power supply, LCD "foreground" gets much brighter and it's hard to read text on LCD... Is there any way to reduce this brightnes thru software but not to effect brightness of text ?
Am powering LED segment display and arduino (with 5V LCD) from same 12V power source but i have test it also with separate 12V supply and situation is the same.
If i lower power supply to 9V then LCD display is working fine but then i need to add separate powers supply for LED segments.
Shortly, is there a way to use 12V power supply to power arduino and to avoid this brightness problem ?
is there a way to use 12V power supply to power arduino
Use a buck converted to get the 12V down to 5V and power it through the 5V pin.
The board regulator is not good at supplying anything much in the way of current due to thermal considerations and it looks like you are getting more than 5V out of it causing you problems with your display.
The USB will not give you 5V, there is voltage drop in its protection circuits in the computer. When you change power you will have to adjust the contrast because of the voltage change. The part you are using is not cheep, lets not destroy it.
I would use a buck converter set at about 8 volts out, use volt meter to set before using. Because of the additional filtering and greater tolerance I am recommend using the Vin to power the arduino. It appears it can also the arcuino +5 to supply power to the GLCD. As far as the LEDs etc it sounds like you can then operate them at 12V. The part you are using is not cheep, lets not destroy it. Without schematics etc this is a best guess. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil
Get out your multimeter and monitor the LCD supply voltage under the different conditions. That will tell you what is happening faster than waiting for us to guess what might be going on.
Perhaps I don't understand your statement .... if I rephrase for my own understanding:
5v Supply (perhaps USB) for both AArduino and LCD = LCD readable
12V Supply to arduino (assuming into Vin or Vraw), LCD supply (backlight led?) is connected to ??? = too bright.
My guess is in case 2 the LCD backlight is connected to 12V resulting in being too bright. Could it be as easy as connecting the backlight LED to the Arduino 5V? Don't know your board or what current the LED draws.